1. Field of the Invention
The present invention relates to a styrene copolymer composition comprising a styrene/n-butyl acrylate copolymer (I) and a block copolymer (II) comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units. More particularly, the present invention is concerned with a styrene copolymer composition comprising a styrene/n-butyl acrylate copolymer (I) comprising styrene monomer units and n-butyl acrylate monomer units in a specific ratio, and a block copolymer (II) having a block configuration comprising at least two polymer blocks (A) each comprised mainly of vinyl aromatic hydrocarbon monomer units and at least one vinyl aromatic hydrocarbon/conjugated diene copolymer block (B), the block copolymer (II) comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units in a specific ratio, wherein the vinyl aromatic hydrocarbon monomer unit moiety of the block copolymer (II) has a specific ratio of the vinyl aromatic hydrocarbon monomer units contained in short segment(s) consisting of 1 to 3 vinyl aromatic hydrocarbon monomer units, wherein the polymer blocks (A) contain a specific amount of a maximum peak molecular weight polymer block exhibiting a maximum peak molecular weight in a gel permeation chromatogram of the polymer blocks (A) and having at least one peak molecular weight in a specific molecular weight range, wherein the polymer blocks (A) have a maximum weight polymer block and a minimum weight polymer block, wherein the weight ratio of the maximum weight polymer block to the minimum weight polymer block is relatively large, and wherein the weight ratio of the styrene/n-butyl acrylate copolymer (I) to the block copolymer (II) is within a specific range. The present invention is also concerned with a styrene copolymer composition comprising a styrene/n-butyl-acrylate copolymer (I) comprising styrene monomer units and n-butyl acrylate monomer units in a specific ratio, and a block copolymer (II) having a block configuration comprising at least two polymer blocks (A) each comprised mainly of vinyl aromatic hydrocarbon monomer units and at least one vinyl aromatic hydrocarbon/conjugated diene copolymer block (B), the block copolymer (II) comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units, wherein the vinyl aromatic hydrocarbon monomer unit moiety of the block copolymer (II) has a specific ratio of the vinyl aromatic hydrocarbon monomer units contained in short segment(s) consisting of 1 to 3 vinyl aromatic hydrocarbon monomer units, wherein the polymer blocks (A) contain a specific amount of a peak molecular weight-having polymer block exhibiting, in a gel permeation chromatogram of the polymer blocks (A), at least one peak molecular weight in a specific molecular weight range, wherein the polymer blocks (A) have a maximum weight polymer block and a minimum weight polymer block, wherein the weight ratio of the maximum weight polymer block to the minimum weight polymer block is relatively small, and wherein the weight ratio of the styrene/n-butyl acrylate copolymer (I) to the block copolymer (II) is within a specific range. (Hereinafter, the block copolymer (II) comprising at least two polymer blocks (A) having a relatively large weight ratio of the maximum weight polymer block to the minimum weight polymer block is frequently referred to as an “asymmetric block copolymer (II)” and the block copolymer (II) comprising at least two polymer blocks (A) having a relatively small weight ratio of the maximum weight polymer block to the minimum weight polymer block is frequently referred to as an “symmetric block copolymer (II)”).
A shaped article obtained from the styrene copolymer composition of the present invention not only exhibits high modulus in tension, high elongation at break, excellent transparency, low natural shrinkage factor (i.e., low shrinkability at ambient temperature) and high resistance to fusion bonding in hot water (for example, low degree of fusion bonding in hot water having a temperature of 85° C.) but also is free from marked occurrence of fish eyes (hereinafter, a “fish eye” is frequently referred to simply as an “FE”). Further, when the styrene copolymer composition of the present invention contains the above-mentioned asymmetric block copolymer (II), a shaped article obtained therefrom has also excellent surface impact strength. On the other hand, when the styrene copolymer composition of the present invention contains the above-mentioned symmetric block copolymer (II), a shaped article obtained therefrom has also advantageously low anisotropy. Further, by incorporating a specific additive into the styrene copolymer composition of the present invention, the occurrence of FE's in a shaped article (such as a sheet or a film) obtained therefrom can be more effectively suppressed.
In addition, a heat shrinkable film obtained from the styrene copolymer composition of the present invention not only is free from marked occurrence of FE's but also exhibits excellent heat shrinkability at relatively low heating temperatures.
2. Prior Art
A block copolymer comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units wherein the vinyl aromatic hydrocarbon monomer unit content is relatively high, has various excellent characteristics, such as transparency and impact resistance, so that the block copolymer has been used for producing injection-molded products, extrusion-molded products (such as a sheet and a film) and the like. Further, a polymer composition comprising the above-mentioned block copolymer and a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has excellent transparency, mechanical properties and shrinkability, so that the composition has been used for producing a sheet, a film and the like.
Conventionally, with respect to the above-mentioned polymer composition, and a sheet or film obtained therefrom, a number of proposals have been made in patent documents. For example, as a composition having not only high tensile strength, modulus and impact strength, but also excellent optical characteristics, elongation property and cracking resistance and the like, Unexamined Japanese Patent Application Laid-Open Specification No. 59-221348 discloses a polymer composition comprising a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer and a copolymer comprising at least one vinyl aromatic hydrocarbon polymer block and at least one polymer block composed mainly of a conjugated diene, wherein the vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has an aliphatic unsaturated carboxylic acid derivative content of from 5 to 80% by weight and has a Vicat softening point of 90° C. or less. As a heat shrinkable film having excellent heat shrinkability at relatively low heating temperatures (for example, at least 20% in terms of a shrinkage factor measured at 80° C.), optical characteristics, cracking resistance, dimensional stability and the like, Unexamined Japanese Patent Application Laid-Open Specification No. 61-25819 discloses a film obtained by stretching a film of a polymer composition comprising a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer and a copolymer comprising at least one vinyl aromatic hydrocarbon polymer block and a polymer block composed mainly of a conjugated diene derivative, wherein the vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has a vinyl aromatic hydrocarbon content of from 20 to 95% by weight and has a Vicat softening point of 90° C. or less. Further, as a transparent, heat shrinkable film having excellent resistance to deterioration with time and impact resistance, Unexamined Japanese Patent Application Laid-Open Specification No. 5-104630 discloses a rigid, heat shrinkable film of a polymer composition comprising a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer and a copolymer comprising at least one vinyl aromatic hydrocarbon polymer block and at least one polymer block composed mainly of a conjugated diene derivative, wherein the vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer has a Vicat softening point of 105° C. or less. This heat shrinkable film exhibits a heat shrinkability in a specific range, as measured in a specific direction of the film. Further, as a polymer composition having excellent transparency, stiffness, and surface impact resistance at low temperatures, Unexamined Japanese Patent Application Laid-Open Specification No. 6-220278 discloses a polymer composition comprising a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene and a vinyl aromatic hydrocarbon/(meth)acrylic ester copolymer resin, wherein the block copolymer has a specific block configuration and a specific molecular weight distribution. Moreover, as a resin composition having excellent transparency and impact resistance, Unexamined Japanese Patent Application Laid-Open Specification No. 7-216187 discloses a transparent resin composition having high strength, which comprises a block copolymer and a vinyl aromatic hydrocarbon/(meth)acrylic ester copolymer, wherein the block copolymer comprises two vinyl aromatic hydrocarbon polymer blocks and two vinyl aromatic hydrocarbon/conjugated diene copolymer blocks.
However, each of the conventional polymer compositions described in the above-mentioned patent documents, each of which comprises a block copolymer comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units and a vinyl aromatic hydrocarbon/aliphatic unsaturated carboxylic acid derivative copolymer, has a problem in that, when the conventional composition is used for producing a relatively thin sheet or film, it is impossible to obtain a sheet or film which not only has high modulus-in-tension, high elongation at break, high surface impact strength, excellent transparency, low natural shrinkage factor and high resistance to fusion bonding in hot water, but also is free from marked occurrence of FE's. In addition, the above-mentioned patent documents do not have any descriptions about how to solve the problem.